1. Field of the Invention
This invention relates to the fields of directional solidification and directionally solidified cast eutectic articles. Articles made according to the present invention are particularly suited for use in high temperature, high stress applications such as blades and vanes in gas turbine engines.
2. Description of the Prior Art
The metallurgical art has long appreciated the fact that differences in microstructure of a given alloy can result in wide differences in mechanical properties. An example of the importance of microstructure in determining mechanical properties is presented in the field of directionally solidified eutectic alloys. U.S. Pat. No. 3,124,452 issued to Kraft and assigned to the present assignee discloses the production of metallic eutectic alloys having a microstructure consisting of parallel second phase plates, fibers, or rods. Directionally solidified eutectic articles possess extremely high tensile strengths in directions parallel to the microstructural alignment, however, such directionally solidified alloys have anisotropic properties and the tensile strengths of such alloys drop off rapidly when tensile forces are applied in directions which do not coincide with the orientation of the microstructure. Directionally solidified eutectics generally have low elongation values and this lack of ductility can be a serious impediment to the successful application of such materials. U.S. Pat. No. 3,790,303 presents one method by which directionally solidified eutectic articles may be produced having a combination of different properties in different parts of the article. This patent discloses an article in which one portion is directionally solidified producing an aligned eutectic structure with a high degree of alignment and a second portion is a nondirectionally solidified portion having a random microstructural orientation. The specific article described is a gas turbine blade in which the root portion which must be attached to the turbine disk has a non-orientated microstructure and this nonorientated microstructure is claimed to have a relatively high ductility so that the blade may be satisfactorily attached to the disk. The nondirectionally solidified root portion of the article has a microstructure which is commonly referred to as dendritic. The airfoil portion of the article has an aligned microstructure with a high degree of alignment and a resultant claimed high yield strength. Consequently the airfoil portion is able to withstand relatively high stresses which are encountered due to centrifugal forces and gas pressures.
Other U.S. patents in the field of directionally solidified eutectics include U.S. Pat. Nos. 3,434,827; 3,528,808; 3,554,817; 3,564,940; 3,671,223 and 3,793,010, all assigned to the present assignee.